why are witches FotM?

I am enjoying my Shadow and have no plans of switching but I was just wondering why all of the top people on both leagues all playing Witches?

Mostly because since launch witches have been somewhat lackluster so not many have played caster builds since then. This is the first time that Witch builds are insanely good. The top players have typically played the better builds at the time so now that witches are actually good many have swapped to them because they have not played them in a while. This is the first time I know I have done a witch build since closed beta other than a summoner. Witches also have a lot of flexibility in their builds especially with the new tree which is why we are seeing so many witch builds, most of the builds you see are very similar..